Meteorites

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What Are Meteorites?

Meteorites are natural objects composed of stone, metal or a mixture of metal and stone that survive their fall from space to land on the Earth. Most meteorites originate from asteroids, a few come from the planet Mars or the moon, and some may even come from comets. Meteorites from asteroids are the oldest objects available for study, dating back to the very first stages of solar system formation, about 4,570 million years ago. The study of meteorites provides a window back in time, so we can learn about the processes that formed and shaped the solar system, and the planets, moons, asteroids and comets it contains, including our own Earth.

Meteorites are divided into three main types on the basis of their composition. Stone meteorites are composed mainly of silicate minerals similar to those found in rocks on Earth. Iron meteorites are composed of iron metal alloyed with nickel. Stony-iron meteorites, as their name suggests, are composed of a mixture of iron-nickel metal and silicate minerals. These different meteorite types are further described in chapter 4.

Meteors, meteoroids, fireballs and meteorites

About 40,000-60,000 t of extraterrestrial material falls on the Earth every year. Before entering Earth's atmosphere these fragments are known as meteoroids, and individual pieces can range in size from tiny, microscopic grains of dust to large bodies, many metres in diameter. The vast majority of the material that lands on Earth is in the form of tiny dust grains, usually less than 1 mm (1/25 in) in size, known as cosmic dust, micrometeorites or interplanetary dust particles (IDPs). Because these objects are so small, they radiate away the frictional heating produced as they pass through the atmosphere faster than they melt, and thus survive intact to land on Earth.

You may have been lucky enough to have seen a meteor or 'shooting star'. These streaks of light are produced by larger objects, perhaps the size of a small pebble. The effects of frictional heating as such objects pass through Earth's atmosphere are great enough to melt and vaporize the material, producing a very rapid, bright streak in the night sky. At some times during the year many meteors can be observed over a period of a few days. These 'meteor showers' are related to the orbits of known comets and occur at well-defined times during the year (see chapter 5). Meteors do not result in meteorites. It is extremely rare for material from a meteor to be recovered on Earth.

If still larger fragments (many centimetres or even metres in size) enter Earth's atmosphere they can produce spectacular light and sound effects as they fall. As they pass through the Earth's atmosphere these are called fireballs. Meteorites may land on Earth as a result of a large meteoroid entering the Earth's atmosphere. A meteoroid travels in space at its cosmic velocity, about 30 km/s (67,500 mph). When it reaches the Earth's atmosphere it is slowed down by friction and these frictional forces also act on its outermost surface, heating it to melting temperature.

Only the outermost surface of the meteoroid melts and the resulting droplets of molten material are carried away as the meteoroid speeds through the atmosphere. (This is unlike meteors, where the entire object is vaporized and destroyed.) The rapid speed of the meteoroid as it enters the atmosphere also generates shock waves (sonic booms), which are often heard as explosions. As the meteoroid is further slowed down by the atmosphere it begins to cool rapidly and its molten surface forms a glassy coating called the fusion crust. Finally, it falls to the ground under the influence of gravity and is now termed a meteorite.

Sometimes one side of the meteorite may be curved or even conical in shape. This is the 'leading edge' of the meteorite, the side that pointed in the direction the meteoroid travelled during its passage through the atmosphere. If the meteorite has tumbled during its flight it will have a more irregular shape, although it should still retain rounded edges. Note that only the outermost surface of the meteoroid melts, while the interior remains cold and unchanged. Meteorites are almost always cold when they land. The very few reports of meteorites that have been recovered almost immediately after landing state that they are cold or slightly warm at most.

Meteorites fall more or less randomly over the Earth's surface and a number of studies have been carried out to estimate how many meteorites land on Earth every year. Some studies use cameras to image the night sky and record the number of observed fireballs, while other studies use the number of meteorites found in desert regions. Estimates indicate approximately 5,000 to 17,000 meteorites larger than 100 g (3 1/2 oz) -- about the size of a golf ball -- fall on Earth every year. However, it is important to remember that 70% of the Earth's surface is covered by water and any meteorites landing in the oceans are unlikely ever to be recovered. Similarly, many parts of the world remain uninhabited or only have sparsely distributed populations, so any meteorites landing in these areas are unlikely to be seen by anyone.

Every year only a handful of meteorites are witnessed to fall and are then recovered -- these are known as 'fall' meteorites. In 2007, for example, there were seven authenticated 'fall' meteorites reported.

In some places, however, meteorites can be preserved or concentrated by natural processes. Most meteorites contain some proportion of metal, and the arid conditions found in the hot and cold deserts help to preserve them by slowing down the weathering and rusting that destroys the meteorites. Hot and cold deserts have provided extremely fertile meteorite hunting grounds, with tens of thousands of meteorites recovered so far. Meteorites that are found after they fell, either by chance or during dedicated collecting trips, are termed 'find' meteorites. Meteorite falls and finds are discussed further in chapter 2.

The study of meteorites -- fireballs and thunderstones

Meteors, fireballs and meteorite falls have been observed and recorded since ancient times. Chinese astronomers recorded meteor showers around 700 BC, and objects including daggers and jewellery made from meteoritic iron have been recovered from Egyptian pyramids, including the famous tomb of Tutankhamen. In 1928, archaeologists excavating a prehistoric Native American settlement near the town of Winona in northern Arizona excavated a 24-kg (53-lb) meteorite that had been carefully buried in a purpose-built crypt, or 'cist'. The archaeological setting suggests that the builders of the settlement treated the meteorite as an important object. However, the Sinagua tribe were only in the area for around 200 years from the eleventh century, and therefore the meteorite is unlikely to have been a witnessed fall.

The oldest meteorite seen to fall, and which is still preserved, is the Nogata meteorite that fell in the garden of the Suga Jinja Shinto shrine in Japan, over 1,100 years ago. Realizing the importance of the object, the shrine priests preserved it in a box as a special treasure. Although no written records exist, the story of the fall was passed down through the generations. In 1922, the head priest contacted a local geologist to ask his opinion on the stone and the geologist declared that it was indeed a meteorite. Then, in 1979, analyzes on a small piece of the stone confirmed this.

The first meteorite fall for which written records exist is the Ensisheim meteorite, which fell on the village of Ensisheim (now in the Alsace region of France) in 1492. A large fireball was observed and, after a large explosion, which caused much fright and excitement amongst the local people, a 127-kg (280-lb) stone fell on Ensisheim. This event was deemed so unusual and important it was recorded at the time in an engraving and was reported to the Emperor Maximillian, who ordered that the stone be preserved in the village church. The stone can still be seen in Ensisheim although it now resides in the town hall.

Between the fall of the Ensisheim meteorite and the late 1700s a number of meteorites were seen to fall and were recovered, although they were considered to be little worthy of scientific study. The modern study and science of meteorites (termed meteoritics) really began in 1768 when a stone that was claimed 'fell from the sky' over the town of Lucé, France, was presented to the Abbé Bachelay. The Abbé collected a number of eyewitness reports of the stone's fall and recovery and also wrote a detailed report, which he then passed on to the French Academy of Sciences. The Academy set up a commission of enquiry, which included the famous French scientist Antoine Lavoisier, to study the stone and the Abbé's report.

The commission undertook the first chemical analyzes of a meteorite. They recognized the presence of iron sulphide and concluded that the stone was simply a terrestrial pyrite (pyrite, chemical formula FeS2, is also known as 'fool's gold'). To account for the eyewitness reports of its fall and the presence of a black fusion crust, the scientists stated that the stone must have been struck by lightning, and consequently, meteorites were commonly referred to as 'thunderstones'. A report by such an eminent group of scientists received wide acceptance among the intellectual elite of Europe. Scientists were unwilling to believe in a phenomenon for which no witnesses of suitably high social rank could be found, and for which no reasonable scientific theories could be provided. Until the end of the eighteenth century meteorites remained to be seen by the scientific community as 'nothing out of the ordinary', and some people even advocated their destruction!

A number of well-witnessed meteorite falls occurred between 1789 and 1803, and these generated many publications of books, articles and pamphlets to explain their nature and origin. In 1794, the German physicist Ernst Chladni published a short book on meteorites. In this book, he described the characteristics of a number of unusual iron masses and descriptions of meteors, fireballs and witnessed falls of stones and irons, and concluded that that they were extraterrestrial in origin.

The year 1794 also marked the fall of the Siena meteorite in Italy. This event generated much interest in the English expatriate community, the majority of whom came from the upper echelons of English society, who were living in Italy or visiting it as part of the 'Grand Tour'. Travellers returning to England brought with them specimens of the meteorite (indeed, a thriving market grew up selling bogus 'fallen stones' to tourists), and fascinating stories regarding the shower of stones over the town.

Just a year later, in 1795, a large stone meteorite fell on farmland at Wold Cottage in Yorkshire, England. The event was witnessed by several people, loud explosions were heard and a young ploughman, John Shipley, witnessed the stone fall through the clouds and on to the ground very close to him. The local landowner, Captain Edward Topham, obtained sworn eyewitness testimonies from three people who had witnessed the fall and from others who had heard the explosions. Early in 1796 a detailed account, submitted by Topham, was published in the local newspaper and he later took the stone to London, where it was exhibited to the public. One of the visitors was the famous British naturalist and President of the Royal Society Sir Joseph Banks, and he obtained a sample of the stone. Banks decided that serious scientific investigation of meteorites should be carried out and so he passed his Wold Cottage fragment, along with samples of other meteorites, to the chemist Edward Howard. Howard discovered that all the samples contained nickel dissolved in iron, a characteristic that was not observed in any terrestrial rocks. Like Chladni, he concluded that an extraterrestrial origin was plausible.

The reports of Edward Howard's analyzes caused many European chemists and mineralogists to re-investigate samples reported to have fallen from the sky. Any remaining doubts, held by the scientific elite of Europe, that the fall of meteorites was a real phenomenon were quashed when a spectacular meteorite shower of over 3,000 stones fell over L'Aigle, France, in 1803. However, where meteorites formed was still a matter of debate, with two main theories competing in the scientific discussions of the early to mid-nineteenth century. Some scientists believed that the stones formed in the upper parts of the Earth's atmosphere, while others believed that they were formed as the result of volcanic explosions on the moon. It is important to stress that at this time the Earth-moon system was considered to be 'closed' (i.e. nothing could be added or lost), and that all atmospheric and weather phenomena (e.g. clouds, rain, hail, meteors, fireballs etc.) were part of this system. Chladni's theory that meteorites were truly extraterrestrial, originating outside the Earth-moon system in cosmic space, was still little believed.

Continued investigation of newly fallen meteorites in the early to mid-nineteenth century revealed that not all meteorites had the same chemistries or textures, or contained the same minerals, which suggested that they could not all originate from one place (i.e. the moon). During the same period, advances in telescope design and manufacture allowed astronomers to study the moon in ever increasing detail, with the result that few astronomers believed that lunar volcanoes were still active. Added to this was the finding that meteorites entered the Earth's atmosphere at cosmic velocities and, therefore, too high a velocity to be of lunar origin. Finally, since their initial discovery in 1801, an increasing number of asteroids were being observed, which were then thought to be the remains of a fragmented planet. It did not seem unreasonable to suggest that some of these fragments could land on Earth as meteorites, and the German scientist Alexander von Humboldt even called meteorites 'the smallest of all asteroids' in 1849.

With meteorites firmly being established as coming from space, their precise origin was still a matter of debate from the mid-1800s until as recently as the 1950s (interestingly the Martian origin of the 'SNC' group of meteorites was only firmly settled in the 1980s, see p.79). An interstellar origin (i.e. coming from outside our solar system) for some meteorites was suggested. However, this theory was dismissed with the precise measurement of velocities and orbits for meteorite parent bodies, which showed that they must have originated within the solar system.

Meteorites, rightly, are now recognized as being rare and precious samples of bodies within our solar system. They record a wealth of information about the birth and growth of the solar system, the formation and history of planets, and the role of impacts in shaping and modifying planets, asteroids and moons, and even record evidence of astrophysical processes that occurred long before our solar system was born.